A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium, in particular to a perpendicular magnetic recording medium that exhibits high recording density and excellent read-write performance. The invention also relates to a method of manufacturing such a medium.
B. Description of the Related Art
As a result of higher performance in personal computers and work stations in recent years, mounted magnetic recording devices having high capacity and small size are required. Thus, a magnetic disk of a recording device needs higher recording density.
Currently practicable magnetic recording systems are “in-plane (or longitudinal) magnetic recording systems,” in which the axis of easy magnetization is parallel to the surface of the magnetic recording medium. To enhance magnetic recording density in this in-plane magnetic recording system, an enhancement of coercivity (Hc) is required as well as a reduction in the product of the residual magnetization (Br) and the thickness (t) of the magnetic film of the recording medium. Accordingly, attempts have been made to decrease the thickness and control the grain size of the magnetic film.
However, the in-plane magnetic recording system involves a problem in that the regeneration output decreases because the demagnetizing field increases with reduction of the bit length and decreases the residual magnetic flux density. In addition, “a problem of thermal fluctuation” becomes pronounced as grain size and thickness of the magnetic film are reduced. For these reasons, it is now deemed technically difficult to obtain a high density magnetic disk under the in-plane magnetic recording system.
To solve these problems and enhance the surface recording density, “a perpendicular magnetic recording system” has been studied. A magnetic recording medium in the perpendicular magnetic recording system is designed so the axis of easy magnetization of the magnetic film aligns perpendicular to the substrate surface. Since adjacent magnetizations in a magnetization transition region are not opposite to each other, the magnetization is stable even with a shortened bit length, and the magnetic flux is not decreased. Therefore, the perpendicular magnetic recording system is suitable for a magnetic recording system for a high density magnetic recording medium.
Contrary to this advantage, a perpendicular magnetic recording medium may exhibit high media noise that results from insufficient precipitation of nonmagnetic elements to a region between magnetic grains in the magnetic layer and thus, the magnetic interaction between the magnetic grains increases. Therefore, achieving high density recording that is accompanied by reduction of the media noise and enhancement of the SN ratio is desired, through development of a material control technology promoting the grain boundary precipitation of nonmagnetic elements.
A perpendicular magnetic recording medium called “a double layer perpendicular magnetic recording medium” is known that comprises a soft magnetic backing layer formed on a nonmagnetic substrate of aluminum or glass, an underlayer formed on the backing layer for perpendicular alignment of a magnetic layer, and a perpendicular magnetic recording layer and a protective layer that are formed on the underlayer. Various multilayer structures have been studied as the perpendicular magnetic layer, including perpendicular magnetic films of a cobalt-based alloy such as Co—Cr, Co—Cr—Ta, or Co—Cr—Pt, multilayer perpendicular magnetic films of Pt/Co and Pd/Co, and amorphous perpendicular magnetic films of Tb—Co and Tb—Fe—Co.
Among the multilayer structures, the multilayer perpendicular magnetic films of Pt/Co and Pd/Co are extensively studied for a future high recording density medium since this type of magnetic film exhibits large perpendicular magnetic anisotropy, high thermal stability, large coercivity, and in addition, a squareness ratio that easily approaches 1.0. These multilayered media, however, generate a large amount of so-called media noise, the reduction of which remains a problem to be solved before practical application.
To reduce the media noise, a granular layer has been applied to the underlayer. Besides this, Japanese Unexamined Patent Application Publication No. 2002-025032 discloses a perpendicular magnetic recording medium that contains boron or oxygen in the multilayer perpendicular magnetic film composed of alternately laminated Co and Pt films or Co and Pd films.
The perpendicular magnetic recording medium disclosed in Japanese Unexamined Patent Application Publication No. 2002-025032, however, causes deterioration of thermal stability of the recorded magnetization since the addition of boron or oxygen to the cobalt layer, platinum layer or palladium layer in the multilayer lamination film causes a decrease of the perpendicular magnetic anisotropy constant Ku (also called simply “Ku” in this specification). It is consequently important to reduce the media noise while keeping the perpendicular magnetic anisotropy constant Ku sufficiently large, to ensure thermal stability.
As is clarified above, the perpendicular magnetic recording medium still involves problems to be solved, although the type of medium is deemed promising for achieving high density recording. Especially, thermal stability must be improved while simultaneously reducing media noise. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.